1. Field of Invention
This invention relates to electrochemical analysis, and particularly to analytical devices in which the ammonia content of sample gases or liquids is measured potentiometrically.
2. Prior Art Statement
Electrochemical methods for the determination of ammonia are typically based on potentiometric measurement of the change in pH caused by diffusion of ammonia into a standard electrolyte solution. Typically, ammonia from a sample liquid or gas enters the electrolyte through a filter or membrane which is selectively permeable to ammonia gas. Ammonia will diffuse into or out of the standard solution until the partial pressure of ammonia is the same on each side of the membrane. Ammonia will react, to some extent, reversibly with water according to the equation: ##EQU1## K being a constant.
If the standard solution contains a dissolved ammonium salt in sufficient amount, the ammonium ion concentration of the solution may also be considered fixed at K'. Thus, the hydroxyl ion concentration may be considered a function of the ammonia concentration, or EQU [OH.sup.- ] .dbd. [NH.sub.3 ] K"
where K".alpha.K/K'. The hydroxyl ion concentration is, of course, measurable by a pH electrode or other pH-detecting device, utilizing the relationships: EQU [H.sup.+ ] [OH.sup.- ] .perspectiveto. 10.sup.-14, and pH = -log [H.sup.+ ].
the partial pressure of ammonia, P.sub.NH.sbsb.3, in turn, is related to the ammonia concentration by Henry's law: EQU P.sub.NH.sbsb.3 = Q[NH.sub.3 ]
where Q is a proportionality constant dependent on the amount of ammonia or ammonia-producing species in the sample, as well as the sample temperature. In practice, therefore, ammonia concentration is determined by preparing a calibration curve of pH against known ammonia standard solutions which have varying amounts of ammonia spanning the range of expected amounts of the sample. The calibration curves are generated on semilog graph paper by plotting the potential readings, in mv., on the linear axis versus ammonia concentration on the semilog axis, which will give a straight-line calibration curve, in accordance with the Nernst equation.
In the past, the standard electrolyte solution has been typically a dilute solution of a very soluble ammonium salt. For example, in the Strickler et al. U.S. Pat. No. 3,649,505, issued Mar. 14, 1972, a combination pH electrode has a hydrophobic, ammonia-permeable membrane which confines a standard electrolyte consisting of 0.01 M to 3M ammonium chloride adjacent the ion-sensitive glass bulb of the pH electrode. To use the electrode, it is simply dipped into the ammonia-containing sample solution, and the change in potential due to the change in pH is measured when the sample solution and standard electrolyte come to equilibrium.
In practice, a calibration curve is usually first prepared, in which the change in potential resulting from immersing or dipping the electrode into a plurality of different ammonia-containing solutions of known ammonia concentration is measured and plotted on semi-log paper against the known concentration. Preparing such curves, then, may require moving the electrode into and out of a series of such known solutions. Acutal measurement of a number of unknown ammonia-containing samples requires moving the electrode into and out of a number of unknown samples, and between sample locations. Unless one is willing to refill the electrode with standard electrolyte solution after each measurement, an obviously inefficient technique, care must be taken that the concentration of the standard electrolyte does not change when the electrode is removed from the ammonia-containing sample solution.
In the Riseman et al. U.S. Pat. No. 3,830,718, issued Aug. 20, 1974, a saturated aqueous solution of an ammonium salt of a strong acid was used as a standard electrolyte to eliminate the possibility of concentration changes due to evaporation of water from the solution. Riseman et al. used ammonium picrate as a preferred internal standard electrolyte. In addition to the benefit that concentration changes by evaporation are eliminated using ammonium picrate as the internal standard electrolyte, Riseman also disclosed a device which may be used to provide accurate determinations of ammonia content of a series of samples without the need for disassembling the electrode and refilling it with standard electrolyte solution.
Federal regulations published by the Materials Transportation Bureau in 49 C.F.R. Sections 170-179 prohibiting the transportation of ammonium picrate on all aircraft, rail cars, and passenger vessels, 49 C.F.R. Section 172.101, with limited exceptions, Section 173.65, make it difficult and expensive to transport ammonium picrate lawfully. Since these regulations took effect in January, 1977, ammonium picrate use as an appropriate internal standard electrolyte has become a practical impossibility.